Wireless, radio frequency communications systems enable people to communicate with one another over long distances without having to access landline-connected devices such as conventional telephones. Such capability is especially important when group activities have to be quickly coordinated, and possibly in dangerous situations or at locations where landline devices are disabled or unavailable. For example, wireless communication is heavily relied upon for livery dispatch, by emergency service agencies (e.g., police, fire, medical), and by the military. In the commercial cellular market, communications between users are primarily point-to-point in nature, e.g., a call is established between two mobile phones. In contrast, wireless communications in the public-safety and similar markets are primarily team or group based. There, a dispatcher talks to a large team in the field, and team members communicate amongst themselves. These markets are currently served using dedicated, private networks that utilize land mobile radio (“LMR”) technologies such as “Project 25,” deployed predominantly in North America, and terrestrial trunked radio (“TETRA”), deployed predominantly in Europe, the Middle East, and Asia. TETRA and Project 25 are both narrowband, digital wireless technologies that support voice and low speed data services. For example, the air interface in a second phase of Project 25 is designed to support four time-division multiplexed channels in 2×25 kHz, or two time-division multiplexed channels in 2×12.5 kHz. Each timeslot carries one voice bearer channel, or a data bearer channel with a peak rate of 9.6 kbit/s.
Because of the context and manner in which they are used, public-safety wireless networks necessarily differ from, e.g., commercial mobile phone networks in a number of ways. As mentioned above, public safety communication is primarily point-to-multipoint, where a dispatcher communicates with a large team in the field, and where members of a squad communicate with others in their squad—a so-called “talkgroup.” Group calls are set up quickly, typically in less than 300 ms, as measured from the time a user hits the push-to-talk button on the handset (commonly called a “pressel”) to the time a channel is granted for the user to start speaking. Additionally, audio propagation delays are low, typically less than 300 ms as measured from the time a user speaks to the time the voice signal is heard on recipient handsets. Public-safety networks also support multiple priority levels, and allow service preemption and queuing for network resources during periods of congestion. Preemptive priority levels are enforced end-to-end. Thus, a user making a priority group call request, for example, can cause the preemption of radio resources assigned to lower priority calls in its serving cell, if necessary, as well as the preemption of radio resources assigned to lower priority calls in the cells serving other group members. Preemptive priority calling features are used to deliver specialized supplemental services such as “man down” alerts.
Existing public-safety wireless networks are typically implemented as “conventional” systems and/or trunked systems. In conventional systems, users control the access to logical channels. Logical channels may be dedicated to specific purposes such as digital voice or data. Users wishing to use these dedicated, pre-provisioned channels manually configure their wireless devices (e.g., radios) for using the desired service. In trunked systems, access to a pool of logical channels is controlled by the network infrastructure. Logical channels are used to carry control messages to and from wireless devices as well as the digital voice and data bearer channels. Control processors allocate channels to users on request, and may give certain users priority over others. Because of the enhanced control features and enhanced spectral efficiencies achieved by trunked systems over conventional systems, trunked systems are being deployed to replace aging legacy conventional systems.
With reference to FIG. 1, a typical trunked public-safety wireless network 20 includes one or more base stations 22 (“BS”) that support the wireless communications interface between the rest of the network 20 and various mobile stations 24, e.g., mobile phones and car-mounted radios. The base stations 22 transmit and receive the logical bearer channels for voice and data services, including transmitting control and system overhead messages for call setup, registration, and other call processing functions. The base stations 22 are connected to a central controller 26 that receives and processes service requests, and that controls the allocation of system resources, e.g., air interface channels, and wireline transmission facilities. For example, the central controller 26 may preempt existing calls to serve those with a higher priority, or queue requests when resources are unavailable. The central controller 26 may be connected to a gateway 28 for a public switched telephone network (“PSTN”) 30, which allows the mobile stations 24 to access PSTN services such as originating and receiving PSTN calls, e.g., calls to public landline phones. Similarly, the central controller 26 may be connected to a data gateway 32 for access to circuit- and packet-switched data services or networks 34 such as the Internet.
Typically, public-safety networks 20 also include a system control computer 36, which serves as the operations, administration and maintenance interface for the central controller 26. The system control computer 36 also provides interfaces for configuration of user options (e.g., access to PSTN gateways, encryption), assignment of users to talkgroups, collection of performance monitoring data, and the like. Further, one or more dispatch consoles 38 may be connected to the central controller 26 through a switch 40, which acts as an interface between the consoles 38 and the central controller 26. The dispatch consoles 38 are computer interfaces used by dispatchers to communicate with the mobile stations 24. A call logging system 42 may be provided to store complete, time-stamped audio transcripts of calls, the identities of the devices involved in the call, or the like.
While existing public-safety wireless networks provide reliable, basic functionality for emergency service agencies and others in need of point-to-multipoint wireless communications, they are mostly appropriate for voice communications. Non-voice data transfer rates are relatively slow, meaning that most existing networks cannot be used for transmitting bandwidth intensive data such as streaming multimedia data or real-time data. The transmission of such information may be beneficial to emergency service and other agencies, e.g., transmitting interactive maps to personnel in the field, or transmitting video feeds from police or military operations. Also, this lack of capacity is contrary to commercial cellular networks, where the trend has been to develop systems with high data transfer rates for wireless Internet access or the like.